This invention relates to thermal management of semiconductor devices and, more particularly, to thermal management of semiconductor devices by direct cooling.
Light-emitting diodes (LEDs) consist of a PN junction formed by two dissimilarly doped semiconductors as shown in FIG. 1. In this junction, one of the semiconductors (p) is doped so the majority carriers are positive (holes), while the other semiconductor (n) is doped so the majority carriers are negative (electrons). By applying an external electric field across the junction, current can be made to flow, and when the holes from the p-type and electrons from the n-type meet at the junction they combine and release a photon of light.
The wavelength of the light depends on the bandgap energy of the materials used in the PN junction. By adjusting the materials used, as well as the doping, a wide range of wavelengths is possible, including wavelengths in the ultraviolet (UV) and infrared (IR) portion of the electromagnetic spectrum. Other electrical components can be made using PN or similar junctions using semiconductors. In all these devices the electrical current and voltage required for operation (whether an LED or not) represent power, and the amount of power in a DC circuit is:P=V×Iwhere P is the power, V is the voltage, and I is the current. In an LED, the conversion from electrical power to optical power is not 100% efficient. Therefore, power that is not released as photons may be absorbed as heat by the LED and the substrate material it is mounted on. The heat generated by the LED leads to a decrease in the output optical power and/or to damage of the device if the temperature is not maintained below a certain level.
While typically less important for operating individual LEDs, thermal management issues tend to be a significant, and often limiting, aspect of the design that goes into large high-power LED arrays. FIG. 2 illustrates the common features of a typical cooling design. LEDs 20, 22, 24 are typically mounted on a substrate 26 that may either act as the heat exchanger or may be mounted to a heat exchanger. Waste heat from LEDs 20, 22, 24 travels by heat conduction through substrate 26 and heat exchanger (if used) where it is picked up by a coolant and removed for release to the environment. The most common feature in typical designs is that heat is removed from the backside of the surface to which LEDs 20, 22, 24 are mounted, which in this example is backside 28 of substrate 26. However, this method of removing heat has disadvantages because it requires the heat to travel through bulk material and (in most cases) several interfaces. Such a system is shown and described in U.S. Pat. No. 6,045,240.
Furthermore, the interface between an LED and the surface to which it is mounted presents heat transfer difficulties. Due to imperfections in materials, the LED and the surface may seem to be in intimate contact while actually being generally separated by a microscopically thin interface. Generally, that interface typically contains one or more undesirable materials, such as, e.g., air. Because air is a very poor thermal conductor, the interface typically is intentionally filled with a selected, thermally conductive material, such as, e.g., epoxy or grease, thereby displacing the undesirable material(s) and improving thermal transport across this interface. Even so, compared with the thermal conductivity of the substrate (and, if used, a heat sink attached to the substrate), this interface may still exhibit relatively low thermal conductivity, leading to undesirably high thermal gradients during the transport of heat energy.
Regardless of the particular characteristics of the thermal path from the backside of the LED into and through the substrate, the heat effectively travels through a thermal circuit that generally has a substantial thermal resistance. Subject to that thermal resistance, the temperature of the LED rises. Moreover, the LED's temperature tends to reach a substantial value in overcoming the thermal resistance and achieving typical heat flow.
Another problem encountered in LED systems that effects thermal management is the efficiency of light transmission. For example, light that is produced within an LED must exit through the LED face. This face represents an optical interface where the index of refraction changes. That change causes reflection of light back toward the LED. Reflected light may be absorbed by the LED, causing it to operate less efficiently. Larger changes in the index of refraction across an interface result in a greater amount of reflection and lessen the amount of transmission. Since LEDs are commonly made of materials with a high index of refraction, the transmission loss at the interface can be significant.
The equation for normal reflection at an optical interface is:
  R  =            (                                    n            2                    -                      n            1                                                n            2                    +                      n            1                              )        2  where n1 and n2 are the indices of refraction across the interface, and R is the power reflection at normal incidence. For example, if light travels through air (index=1) and strikes a glass surface at normal incidence (n=1.5), then the amount of reflection will be 0.040, or 4%, and the amount of transmission will be 1-R (for lossless interfaces) or 96%.
The material used in the construction of LEDs typically has a very high index of refraction. For example, Cree, Inc. (Durham, N.C.) makes an LED that is constructed of silicon carbide, which has an index of refraction of about 2.8 at short wavelengths near the upper end of the ultraviolet as seen in FIG. 3. With such a high index of refraction, the reflection at an interface of SiC and air is 22.4%, resulting in a transmission of only 77.6%.
What is needed is a system to provide efficient thermal management of a semiconductor device. In another aspect, what is needed is a system that efficiently manages heat generated by an LED so as to increase the operational efficiency and/or lifetime of the LED. In another aspect, what is needed is a system that efficiently manages heat generated by an LED so as to-provide for enhanced transmission of light through the LED interface.